This invention relates to opening and closing apparatus for rolling up or unrolling a flexible screen such as plastic film, sheet material, etc., of the type which is stretched in a greenhouse or the like to form a partition.
Generally, when a film or the like of continuous length is rolled up or unwound by a winding rod which rotates at a constant speed, the roll diameter increases or decreases in accordance with the amount of the film wound up or unwound, so that the winding or unwinding speed is variable even when the winding speed of the winding rod is constant. In such a case, in order to maintain the speed at which the film or the like which is wound or unwound at a constant rate, it is necessary to adjust the rotating speed of the winding rod which rotates at a fixed position. However, in order to change the rotating speed of the winding rod in accordance with the increase or decrease of the roll diameter by using driving means such as an electric motor for rotating the winding rod, a complicated speed change mechanism is required as well as adjustments thereof which are quite difficult. Further, since the rate at which the roll diameter increases or decreases also varies according to the thickness of the film or the like used, fixing the rate of change of the speed of the driving shaft greatly restricts the scope of application of the apparatus and such is therefore not practical.